Which of the following best defines 'QoS'?

Quality of Service (QoS) is fundamentally a strategy designed to manage network resources in order to optimize application performance. This is critical in environments where multiple applications might compete for limited bandwidth and data resources. QoS involves various techniques for prioritizing certain types of traffic over others to ensure that essential applications receive the necessary bandwidth and latency they require to function effectively.

For instance, in a network supporting voice, video, and data, QoS allows for voice packets to be prioritized to ensure clear communication without delays, while less time-sensitive data traffic can be processed with lower priority. This management of resources ensures that user experiences remain optimal, especially for applications requiring real-time data transmission.

The other choices do not capture the essence of QoS accurately. While a measure of network traffic relates to monitoring performance, it does not address how traffic is managed or prioritized. A technology that increases data speed suggests a focus on acceleration rather than resource management. Lastly, a tool used for network diagnostics pertains to troubleshooting and analyzing issues rather than the systematic management of application performance. Thus, strategy for managing resources for application performance is the most fitting definition of QoS.